1. Field of the Invention
The invention relates to noise-reducing shims for disc brake systems, and more particularly relates to shims for mounting on the brake piston.
2. Description of the Related Art
In a disc brake caliper, a hydraulically actuated piston forces, by action and reaction, a pair of opposing brake pads to pinch a rotor attached to a vehicle wheel. Brake pads have a stiff backing plate with friction pads affixed to one side. Shims of a thin material are often used on the side of the plate contacted by the piston. The shim's function is to reduce the frequency of occurrence of a loud and annoying squeal noise from what are otherwise mechanically perfect brakes. At least some of the noise comes from the fact that, to prevent jamming, the plate has some freedom of radial movement within the caliper so that the rotor can be freed, while the piston has little freedom of movement.
In braking, the rigid rotor is frictionally engaged by the hard brake pad (which is in turn supported by its hard backing plate). The brake pad can frictionally slide to some degree against the rigid piston and caliper.
High frictional forces are generated during this sliding movement which can cause squeal. Like a stick of chalk forced at an angle across a rigid board, friction can lead to very severe vibrational noise. As well, these recurring forces can lead to fretting and chipping of the piston rim. A great many designs of shims have been tried over the years to fit between the piston and the plate to reduce such friction and resulting squeal. None have been entirely satisfactory.
Brake squeal is a very expensive problem for car and brake manufacturers as customers invariably want the noise remedied under warranty even though the brakes are otherwise perfect.
Shims are typically designed to fit against the backing plate of the brake pad. Almost every separate vehicle model has a unique backing plate size and shape. This requires large and therefore expensive inventories.
Piston-side shims are less well-known, but have certain advantages. One advantage is that there are fewer variations in piston size and shape across vehicle models.
However, existing piston-side shims are bulky and have undesirably complex manufacturing requirements. Further, such piston-side shims are not designed to optimize smooth sliding potential against the brake backing plate, and accordingly may not be effective in reducing vibrational noise. It would desirable to allow the shim to slide smoothly against the backing plate to reduce the undesirable friction. At the same time, the shim must be maintained in a secured and well-located state on the piston edge to prevent the piston edge from becoming exposed on one edge when contacted with the backing plate (i.e. to prevent scraping and fretting of the piston edge which can lead to damage).